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Friday, May 22, 2015

Modulating movement intention and the extended present with tDCS.

Douglas et al. do a fascinating bit of work on the 'extended present' in which our brains function, during which our experienced intention to make a movement actually comes ~200 milliseconds after motor cortex signals initiating the movement (the famous Libet experiment showing we are 'late to action'). Conscious intention, or volition, provides the foundation for our attributing agency to ourselves, and for society attributing responsibility to an individual. A distorted sense of volition is a hallmark of many neurological and psychiatric illnesses such as alien hand syndrome, psychogenic movement disorders, and schizophrenia.

Conscious intention is a fundamental aspect of the human experience. Despite long-standing interest in the basis and implications of intention, its underlying neurobiological mechanisms remain poorly understood. Using high-definition transcranial DC stimulation (tDCS), we observed that enhancing spontaneous neuronal excitability in both the angular gyrus and the primary motor cortex caused the reported time of conscious movement intention to be ∼60–70 ms earlier. Slow brain waves recorded ∼2–3 s before movement onset, as well as hundreds of milliseconds after movement onset, independently correlated with the modulation of conscious intention by brain stimulation. These brain activities together accounted for 81% of interindividual variability in the modulation of movement intention by brain stimulation. A computational model using coupled leaky integrator units with biophysically plausible assumptions about the effect of tDCS captured the effects of stimulation on both neural activity and behavior. These results reveal a temporally extended brain process underlying conscious movement intention that spans seconds around movement commencement.